Mammals didn’t have to wait for the dinosaurs to die out

A study recently published in Nature challenges the prevailing wisdom that the extinction of the dinosaurs paved the way for an explosion of mammalian diversity. By studying the fossils of a group of mammals called multituberculates, the researchers have cast doubt on the traditional view of dinosaurs constraining small, shrew-like mammals to a secondary role as nocturnal insectivores. Instead, this group seems to have diversified millions of years before the dinosaurs died out.

Multituberculates were a widespread group of rodent-like mammals which first appeared in the fossil record about 165 million years ago and died out about 35 million years ago. For the majority of this time they co-existed with the dinosaurs, which went extinct around 65 million years ago. The researchers were able to estimate the diversity of the multituberculates during this period by using fossilized teeth to make inferences about their diet and morphology. Since enamel its quite tough, teeth trend to fossilize well and so are a mainstay of palaeontology. This is particularly true for early mammals, whose small, delicate bones rarely became fossils.

The researchers made high resolution scans of teeth from more than 40 different multituberculates and then used geographic information system (GIS) software borrowed from the world of cartography to generate a detailed topographical map of the teeth. By carefully determing the shape of teeth, the researchers were able to measure the number of patches facing the same direction, called the “Orientation Patch Count” (OPC), which they used as an esimate of dental complexity. Dental complexity can can be used to learn about an animal’s diet, since different kinds of food require teeth of different shapes. For example, carnivores, who tend to have sharp, simple teeth suited for slashing and piercing, would have a low OPC; by contrast, a herbivore would have a high OPC, since their teeth are more complex and rugged in order to crush and grind their food. Measuring the OPC of fossilized teeth thus enabled the researchers to get an idea of what kind of food the multituberculates were eating. Furthermore, the variability in OPC at any point in time gives an idea of the diversity in dirty and morphology within the group.

From their first appearance until about 84 million years ago, the multituberculates had a relatively low average OPC with very little variation. Based on a comparison with values in modern mammals, it seems likely that these animals had a carnivorous or animal-rich omnivorous diet, such as a mixed diet of insects and fruit. OPC values then increased sharply between 84-66 million years ago, with some taxa reaching levels consistent with plant-dominated omnivory or herbivory in modern mammals. At the same time, the variability also increased, reflecting greater morphological diversity. Both diversity and OPC values continued to increase through the extinction of the dinosaurs, reaching their peak sometime between 66-62 million years ago. OPC levels declined between 62-49 million years ago, though variability remained high; following this, only one genus (with quite a low OPC) is known before the multituberculates went extinct.

These data tell a story of early multituberculates being a relatively homogenous group of carnivores for the first half of their existence, followed by rapid diversification 20 million years before the dinosaurs went extinct. This explosion in diversity was concomitant with a switch to a plant-rich diet and continued even through the mass-extinction event that wiped out the dinosaurs. This interpretation is also supported by estimates of generic richness and body size.

What change allowed these mammals to diversify millions of years before the dinosaurs disappeared? The researchers suggest that it may have been thanks to new opportunities presented by flowering plants, which underwent an increase in ecological diversity and abundance around the same time, as well as an increase in leaf-vein density. These plants were probably soft green plants (herbaceous) and may have had less effective herbivore defenses compared to other seed plants, making them an attractive, abundant source of food. The evolutionary and ecological diversification of the angiosperms may have provided a range of new niches into which the multituberculates could spread.

In addition to teaching us something about the history of life on Earth, this story also provides a much more interesting and nuanced view than simplistic narratives about which group of animals was dominant or “ruled the Earth”. Instead, this is a story about an adaptive radiation into new ecological niches, which is a far more common pattern in evolution. The fact that the diversity of the multituberculates and their food remained high through the mass-extinction that wiped out the dinosaurs nicely shows how ecologically selective these events can be. This is a great study and an excellent example of powerful yet simple science.